Field of the Invention
The present invention relates to a hydrocarbon gas sensor that detects a hydrocarbon gas in a measurement gas.
Description of Related Art
There has been already known an apparatus capable of measuring a total concentration of inflammable gases in a measurement gas containing, as inflammable gases, carbon monoxide (CO), hydrogen (H2), and hydrocarbon (HC) such as propane (C3H8), or the concentration of a hydrocarbon gas in the inflammable gases (for example, see Japanese Patent Publication No. 3450084). Combustion exhaust gases in, for example, an internal combustion engine, an external combustion engine, and a combustion path are illustrated as examples of the measurement gases.
In outline, the apparatus disclosed in Japanese Patent Publication No. 3450084 is mainly composed of zirconia being an oxygen-ion conductive solid electrolyte and includes a first processing zone and a second processing zone each defined from the outside. This apparatus is configured to apply, between an opening portion facing the outside and the first processing zone and between the first processing zone and the second processing zone, a predetermined diffusion resistance to a gas atmosphere flowing therebetween. In this apparatus, the measurement gas containing inflammable gas components is first guided to the first processing zone from the outside of the apparatus with a predetermined diffusion resistance, and then, its oxygen partial pressure is reduced such that the inflammable gas components are not substantially burned, through an oxygen pumping action by a first electrochemical oxygen pumping cell in the first processing zone. After that, the measurement gas is guided to the second processing zone with a predetermined diffusion resistance. Then, in the second processing zone, the inflammable gas components existing in the atmosphere are burned when being supplied with oxygen through the oxygen pumping action by a second electrochemical oxygen pumping cell. The pump current flowing through the second electrochemical oxygen pumping cell or the voltage generated between the electrodes of the second electrochemical oxygen pumping cell is detected in this burning, to thereby obtain a concentration of the inflammable gas in the measurement gas based on the detected value.
A selective oxidation catalyst, which does not oxidize a hydrocarbon gas but selectively oxides inflammable gases except for hydrocarbon gases, such as a carbon monoxide gas and a hydrogen gas, is placed between the opening portion and the first processing zone, so that the hydrocarbon gas alone is guided to the second processing zone As a result, the concentration of the hydrocarbon gas can be obtained.
The above-mentioned concentration measurement technique with the apparatus disclosed in Japanese Patent Publication No. 3450084 is capable of obtaining a desired concentration with high accuracy in case where the measurement gas contains no water vapor (for example, in a case of a binary system of oxygen and inflammable gas).
Actually assumed as the measurement gas in the above-mentioned concentration measurement method is, however, a combustion exhaust gas mainly in an internal combustion engine, external combustion engine, or combustion path. Such a combustion exhaust gas contains approximately 2% to 12% (at a volume percentage) of water vapor generated through burning of the inflammable gas.
If the above-mentioned technique is applied to a measurement gas containing not only oxygen and inflammable gas but also water vapor to control the oxygen partial pressure to such a small value as not to substantially burn the inflammable gas in the first processing zone, the water vapor is decomposed, generating a hydrogen gas being an inflammable gas. The generated hydrogen gas flows into the second processing zone together with the inflammable gas that is originally a measurement target, which has been contained in the measurement gas, and is then burned in the second processing zone. In other words, a larger amount of oxygen than required for burning the inflammable gas being an original measurement target is consumed, resulting in a large error in the concentration value obtained. For example, for the oxygen partial pressure of 10−14 atm to 10−20 atm in the first processing zone, the concentration of the hydrogen gas to be generated through decomposition of water vapor may reach several %.
Japanese Patent Publication No. 3450084 also discloses the configuration in which hydrogen in the measurement gas is pumped out by a proton pump provided, and then, the measurement gas is guided to the second electrochemical pumping cell. In this configuration, however, a solid electrolyte formed of a material different from zirconia is used for only the proton pump, resulting in a complex process. Also, the complex configuration is disadvantageous in costs.